A wide variety of pure phase materials such as polymers are now readily available at low cost. However, low cost pure phase materials are somewhat limited in the achievable ranges of a number of properties, including, for example, electrical conductivity, magnetic permeability, dielectric constant, piezoelectric coefficients, refractive index, luminescence and others. In order to overcome these limitations, composites can be formed, in which a matrix is blended with a filler material with desirable properties. Examples of these types of composites include the carbon black and ferrite mixed polymers that are used in toners, tires, electrical devices, and magnetic tapes.
The number of suitable filler materials for composites is growing, but the process is still limited. In particular, difficulties in fabrication of such composites often arise due to issues of interface stability between the filler and the matrix, and because of the difficulty of orienting and homogenizing filler material in the matrix. Some desirable properties of the matrix material (e.g., rheology) may also be lost when certain fillers are added, particularly at the high loadings required to achieve percolation using conventional fabrication techniques. In making such composites, a sufficient amount of filler must be added to overcome the percolation threshold, the critical concentration of filler at which the polymer will begin to acquire the property of the filler (e.g., in the case of electrically conducting fillers, the percolation threshold is the concentration of filler at which the composite will conduct an electrical current). Beyond this threshold, the property generally increases markedly as additional filler is added. It is believed that at the percolation threshold, uninterrupted chains of filler particles first appear in the system. The addition of still greater amounts of filler produces a correspondingly higher number of uninterrupted chains, which results in still higher levels of the desired property until the property levels out to that of the properties of the filler.
For instance, electrically insulating polymers can be made electrically conductive via the addition of electrically conductive fillers, such as carbon fibers, carbon blacks, carbon nanotubes or metal fibers. Electrically conductive polymer systems are prized as materials for electromagnetic shielding in electronics applications and as materials used in the fabrication of structures to which paint may be applied using electrostatic painting techniques. Certain fillers such as carbon fibrils are high cost materials. Often the filler material is more expensive than the matrix material, particularly at known achievable percolation thresholds. Additionally, the use of such fillers may degrade other important physical characteristics of the material such as its impact strength. Some electrically conductive fillers have a more pronounced negative effect on certain material's physical properties than others, but nearly all polymer systems incorporating them suffer a degradation of impact strength, or certain other physical properties not related to conductivity, relative to the unfilled polymer systems. In many instances, the desired level of electrical conductivity cannot be obtained without sacrificing at least some part of the material's inherent impact strength or other properties.
Therefore, it would be desirable to maximize the electrical conductivity enhancing effect of the conductive filler while minimizing the filler cost to achieve the desired electrical conductivity by reducing the percolation threshold for the filler. Further, it would be desirable to maximize the electrical conductivity enhancing effect of the conductive filler while minimizing the resultant change or loss in other matrix properties. The ability to fabricate composites having the desirable properties of a filler material, by using a lower amount of filler material and the ability to control the amount of the property acquired by the composite material would significantly expand the scope of manufacturable composites.